Space exploration aficionados experienced the thrill of anticipation in the hours before President Trump’s speech Tuesday night, with advance word that he was going to call for a return to the human exploration of space.

Sure enough, in his closing words Trump declared that for a country soon to celebrate its 250th anniversary, “American footprints on distant worlds are not too big a dream.”

Trump’s brief, offhand comment had the tone of an impulsive notion that, like so many of his other policy pronouncements, won’t get any follow-through. Let’s hope so, because the idea of sending humans to explore distant worlds is loopy, incredibly wasteful, and likely to cripple American science rather than inspire it. And that’s assuming that Trump’s notion doesn’t have the ulterior motivation of diverting American scientists from their Job One, which is to fight climate change right here at home.

It's hard to see any particular reason or purpose in going back to the moon or indeed sending people into space at all.— Astronomer Royal Martin Rees

The idea of sending humans back into planetary exploration, with Mars as the prime target, has been a crowd-pleasing dream of presidents ever since Gene Cernan became the last American to set his footprints on the moon in 1972. As the author Ken Kalfus toted up the record, during the Reagan administration a congressional commission called for a return to the moon by 2005 and a Mars landing by 2015; George H.W. Bush declared that “the American flag should be planted on Mars” by the 50th anniversary of the first Apollo moon landing (2019); and George W. Bush moved the deadline out to a moon landing by 2020 in preparation for a leap to Mars “and other destinations.”

Barack Obama canceled the Constellation program that might have fulfilled the latter Bush’s dream, but eased the pain by calling for sending astronauts to an asteroid by 2025, orbiting humans around Mars by the mid-2030s, and landing them on the surface soon after that — within his own lifetime.

The romance of human space exploration doesn’t belong only to politicians. It’s been exploited, for example, by the industrialist Elon Musk, who last year unveiled a vision of human colonization of Mars to turn humankind into a “multiplanet species” to safeguard against an “extinction event” on Earth. Musk’s private rocket company, SpaceX, recently announced that it has taken deposits from two customers for orbital voyages around the moon.

The exhortations by presidents share several assumptions. One is that the manned moon exploration programs Mercury, Gemini, and Apollo have yielded stupendous returns in science, engineering, and economics — and that the exponentially more challenging voyage to Mars will yield exponentially greater benefits. Another is that humans are needed to perform some functions in space that robots can’t do. A third (seldom voiced directly) is that only the drama and romance of human spaceflight can attract the public interest and support needed for such an expensive program. At the peak of the space race, NASA commanded fully 4% of the federal budget, a share that could only be sustained by tapping into public excitement.

None of these assumptions is warranted, even though the scientific and economic returns from the space programs are invariably invoked as articles of faith. Typical is this claim made in October by two Trump campaign advisors, former GOP Rep. Robert S. Walker of Pennsylvania and UC Irvine economist Peter Navarro: “Our past investments in space exploration have produced brilliant returns for our economy, our security and our sense of national destiny.” In their article, Walker and Navarro don’t actually mention any specific economic returns, brilliant or otherwise. That’s unsurprising, because it’s hard to identify any that would not have been produced by an unmanned moon program.

The presidential visions of human space exploration all hark back, of course, to President Kennedy’s 1961 call to send a man to the moon and bring him back alive by the end of that decade, a quest that was fulfilled. That was a different time, however: America was in the heat of technological and economic battle with the Soviet Union, the 1957 Sputnik flight still stung, and the Soviets had recently sent cosmonaut Yuri Gagarin in orbit around Earth. Back then we were all vulnerable to the cult of the astronaut; as a kid I knew the names and personal stories of all the original seven Mercury pilots. Today few can summon up the names of shuttle astronauts with the exception of Christa McAuliffe, who is recalled chiefly because of her tragic end on the shuttle Challenger. Today’s sense of the limitations of public funding of science and heightened awareness of competing demands on the federal budget closer to home didn’t exist in 1961.

Are humans necessary for space exploration? Less now than ever, with the vast advances in robotics achieved since the last moonwalk in 1972. Astronomers and other scientists long have been skeptical of the need for human exploration. In 2010, Astronomer Royal Martin Rees of Britain said, “The practical case gets weaker and weaker with every advance in robotics and miniaturization. It's hard to see any particular reason or purpose in going back to the moon or indeed sending people into space at all."

As physicist Steven Weinberg observed more than a decade ago, placing humans on a space mission makes it so much more expensive than an unmanned flight that some elements of the mission get jettisoned — and those are almost always scientific projects. The public obviously considers the human participants to be indispensable, so much so that a loss of life can almost destroy a space program, as happened with the space shuttle program after two human catastrophes. Accordingly, protecting human lives and health becomes paramount; the cost of those arrangements will be much greater on a Mars flight, which is estimated to take as long as nine months.

Weinberg makes short work of the best example made for the necessity of humans in spaceflight. This is the series of repair missions on the Hubble Space Telescope performed by shuttle crews, the last time in 2009. The Hubble is one of several orbiting observatories that have added immeasurably to our knowledge of distant space. But because it was launched by the shuttle, it was also uniquely expensive. Weinberg quotes Riccardo Giacconi, the former director of the Space Telescope Science Institute, as estimating that had the telescope been launched by unmanned rockets instead of the shuttle, seven Hubbles could have been launched for the same price as the one we got. “It would then not have been necessary to service the Hubble,” Weinberg writes; “when design flaws were discovered or parts wore out, we could just have sent up another Hubble.”

What really underlies the lure of human space exploration is its romance and drama, fostered in part by decades of popular culture, including “2001: A Space Odyssey,” “Star Trek” and “Star Wars,” and “The Martian.” The characters in these space operas are our heroes, but what’s often overlooked is that many of these are disaster stories. The thrill we feel from the interplanetary rescue of the stranded astronaut of “The Martian” obscures the more fundamental question of why he had to get stranded up there in the first place.

Among the dangers of cavalier calls for publicly-funded human space exploration is that monumental Big Science programs like the space race tend to suck resources away from any science left on the outside looking in. A multitrillion-dollar program to put an American on Mars, endorsed by a president, will get first call on the federal budget, leaving programs aimed at disease cures, chemistry, and physics far behind.

In the current political climate, the biggest threat is to Earth science, which is increasingly devoted to climate change. It may not be a coincidence that conservatives in Congress have been systematically trying to cut NASA’s Earth Science budget in favor of planetary exploration, albeit unmanned exploration. They argue that the goal is to refocus NASA on its traditional mission. But that’s a smokescreen, because research in climate science has become a major part of NASA’s mission. They’re really displaying their hostility to research that could undermine the fortunes of their patrons, the fossil fuel industry. If Trump’s call for manned planetary exploration is another puff of that smokescreen, it would hardly be surprising.

Sending humans into space would give Americans a sense of mission and grandeur, but that’s mostly a sign of civic immaturity. Take the same sums and spend them on curing disease — whether the biological malady of cancer or the social maladies of poverty and hunger — and pride will surely follow. Keep the astronauts at home, and there will be much more money available to send robots farther out than humans could ever go, and to bring back immeasurably more knowledge.